1. Field of the Invention
The invention, in general, relates to the readjustment of the polarization drift caused by optical fibers, e.g. for polarization-encoded q-bits.
The invention, in detail, relates to a method and a device for the adjustment control of a polarization drift in the transmission of a polarization-encoded optical signal from a transmitter via an optical waveguide to a receiver.
2. Description of Related Art
Polarization is used only to a limited extent for classic optical communication, because this property of light is strongly influenced in optical fibers (light guides, optical waveguides). Because of very small deviations of the normally round fiber geometry, double refraction with a rapid axis and a slow axis is formed, which causes the light with the respective polarizations to be transported differently rapidly. The respective axis indicates the direction of the electric field vector and is always oriented perpendicularly to the direction of propagation. The positions of the axes within the fiber are undefined in the sense that both the rapid and the slow axes may point in any direction, yet both axes will always extend perpendicularly to each other.
If polarized light is incoupled exactly in the axial direction of a fiber, it will typically retain its polarization over hundreds of meters until the axes will slightly drift in their positions. The starting positions of the axes are, however, unknown. The luminous power is split onto the slow and the rapid axes as a function of the coupling angle and will loose its original position within a few centimeters of the propagation distance: If the different propagation speed causes the divergence of the wave packs already by portions of the wavelength, the wave packs will still be able to superpose, yet a different polarization will adjust. The result of such a change in polarization is that the polarization will no longer be predictable within the shortest fiber paths. In addition, this drift is strongly dependent on the mechanical pressure exerted on glass fibers and, naturally, will thus also depend on time and temperature.
Despite these circumstances, it is possible in conventional transmission technology to virtually double the capacity in glass fibers by the aid of polarization. The information transmitted in one of the bases is independent of the information of the other base, and in this scheme it is made sure that the two bases will not interfere with each other. In doing so, two identical optical laser diodes (having equal radiation wavelengths) are modulated with the information of two transmission channels, the only difference chosen being that the respective polarizations are perpendicular to each other. Using already commercially available devices for readjustment (cf.: http://www.thorlabs.com/thorProduct.cfm?partNumber=PL100S www.bfioptilas.de; http://www.generalphotonics.com/PolaStay.htm), it can be ensured that a polarization radiated into a base (e.g. horizontally or vertically) will still be received in that polarization at the receiver. The second channel would automatically be in a polarization rotated by 90°.
A polarization-encoded, classic transmission system is conceivable, in which a bit value, e.g. “0”, is to be assigned to a horizontal radiation and the other bit value, e.g. “1”, is to be assigned to a vertical radiation. In a manner similar to the two independent information channels mentioned above, the readjustment of the polarization would have to be used in order that a transmitted bit value would have the same meaning to both the transmitter and the receiver. Such an encoding is avoided in practice, because the discrimination between high/low amplitudes or different phases is transmitted to a reference signal in a much more interference-free manner.
By contrast, it would be readily feasible to utilize such an encoding in quantum technology. The generation and measurement of the quantum information encoded with a polarization is a routine procedure already well controllable, and for this reason polarization-encoded quantum bits (q-bits) are highly attractive for quantum applications like quantum cryptography (QKD for “quantum key distribution”). The technique presented here, in particular, deals with the question of the distribution and stabilization of polarization-encoded photons to enable the reading of the information transmitted via an optical cable also at the receiver, but is not limited thereto and, above all, not to the application for telecommunication wavelengths in the wavelength band of about 1310 nm or 1550 nm, although it is of particular advantage in this field.
In a typical arrangement used in quantum communication, it is mostly photons which are chosen to be used as individual information carriers. They exhibit the least interaction with the carrier medium (e.g. glass fiber or air in free-space propagation) and retain their polarization and, hence, the stored information over long distances. The quantum channel is located between the generation (preparation), on the one hand, and the measurement, on the other hand. If this channel is completely or partially comprised of optical glass fibers, a strong transformation of polarization will have to be expected in the same. Yet, this can be reversed again as long as no depolarization is caused by polarization mode dispersion or any other effect. As long as this does not happen, the technique presented here is able to restore the original state. This is exactly what is essential therefor, namely that all of the polarization states at the input be imaged to the output as precisely as possible:
As opposed to binary, conventional bit values (comparable with a light switch), the encoding of information in quantum technology is effected by q-bits. These constitute a superposition of two states. It can be readily accomplished through polarization. Like with the above-mentioned, but not used polarization-encoded, classic method, one photon has two polarization states that can be produced. In addition, a superposition of the states is also of particular importance. This constitutes an essential difference to the classic type of information processing, because the superposition of states will only make sense in quantum technology.
Needless to say that in quantum communication it is, in particular, to be taken care that both the states and this superposition be transported in an errorless manner. This cannot be achieved by the above-mentioned commercial systems, because there the field of application was the maintenance of a base, e.g. the (H,V) base (horizontal/vertical polarization). By contrast, quantum technology requires an automatic control also controlling a second base with sufficiently high accuracy. In this respect, it is possible to use the second base with linearly polarized light, the (+/−)-base (linearly +45°/−45° polarized) or the circularly oriented (L/R)-base (left/right hand circularly polarized). If a second base is controlled besides the first base, all superposition states and, hence, also the third base are fully defined. It is no longer necessary to control this third base as well. It should be mentioned that it is known per se from Cheng-Zhi Peng, Jun Zhang, Dong Yang, Wei-Bo Gao, Huai-Xin Ma, Hao Yin, He-Ping Zeng, Tao Yang, Xiang-Bin Wang and Jian-Wei Pan, “Experimental Long-Distance Decoy-State Quantum Key Distribution Based on Polarization Encoding”, Phys. Rev. Lett. 98, 010505 (2007) to use two attenuated laser diodes for two bases integrated in the system. The used single photons, however, involve a long communication time in the order of minutes and, therefore, cannot be used for the readjustment of typical polarization drifts (to be clearly recognized from FIG. 2).
Every possible polarization state can be unambiguously represented by said three independent bases, since it is possible to compose every polarization to be analyzed of the polarizations of a base (so-called eigenstates). Horizontal polarization is, for instance, produced if identical +45°- and −45°-polarized radiations are superposed. If the radiated polarization is chosen exactly in the direction of the bases (so-called base eigenstate), the respective analyzer will pass on the whole power into an arm. If, for instance, horizontal polarization is analyzed in the (H,V)-base, the H-detector will indicate a maximum and the V-detector will be in the minimum. However, if an eigenstate of another base falls on the analyzer, the radiation will be evenly distributed. Horizontal polarization produces identical starting values amounting to exactly half of the maximum value both in the (+,−)-base and in the (L,R)-base. A polariometer is used to analyze an unknown incident radiation in all three bases. In doing so, every possible, pure polarization state is unambiguously determined.
Even with an initially well equipped overall transmission system (with the total polarization drift of the fiber being compensated), temperature changes and vibrations will cause a time-dependent deviation of the polarization, which will have to be tracked. If the glass fiber runs aerially over long distances, changes typically occurring as slow drifts will result from similar temperature changes of the day/night cycle. If fiber paths extend on bridges, a plurality of differently slow and rapid effects may couple onto the fiber. The technically most challenging application comprises overhead lines on e.g. high-tension line poles, since uncoordinated pendulum movements caused by wind may lead to strong deviations from the initial state. Because of the most diverse applications, it is suitable to separate the transmission path from the remaining quantum array and introduce separate readjustment. In doing so, it is of particular importance to realize the required timings so rapidly that the readjustment will react more rapidly than the typical time constants of the interference will influence the deviation from the desired polarization.
An important issue in the configuration of a polarization control is that the influence by the control loop on the individual photons as useful signals be as little negative as possible. A completely independent operation would be desirable. The solution to route a channel with another wavelength over the same fiber involves the drawback that the dependencies of the polarization drift of the two wavelengths differ too strongly when using the telecommunication standard CWDM (“coarse wavelength division multiplexing”). The wavelength raster of 20 nm used in that case is too coarse. An option would be the use of closely adjacent wavelengths that would be available in DWDM (“dense wavelength division multiplexing”). Even then it will not be guaranteed, due to a slightly different dependency of the control element, that the readjustment signal and the quantum signal do have the same polarities. The invention is, therefore, based on that the readjustment signal is to have the same wavelength as the useful signal.